Compound varactor

ABSTRACT

The present disclosure provides a method for fabricating a compound varactor. The method includes steps of depositing a collector layer, depositing a first base layer arranged in a first plurality of parallel fingers directly onto the collector layer, and depositing a second base layer arranged in a second plurality of parallel fingers that are interleaved with the first plurality of parallel fingers directly onto the collector layer.

This application is a Divisional filing of U.S. utility patentapplication Ser. No. 14/485,532, filed Sep. 12, 2014, the disclosure ofwhich is incorporated herein by reference in its entirety.

FIELD

Embodiments of the present disclosure relate generally to the field ofcircuits, and more particularly to reverse-biased diodes (varactors).

BACKGROUND

A diode under reverse bias may exhibit a capacitance that variesinversely with the applied voltage. A component that behaves in thismanner, e.g., as a variable capacitor, may be termed a varactor. Thevariable capacitance of the diode may be used for “tuning” electricalcircuits. Generally, semiconductor varactors may have a wider tuningrange (e.g., capacitance variance) and lower control voltagerequirements than dielectric varactors realized on materials such asbarium strontium titanate (BST). However, the semiconductor varactorsmay typically exhibit a lower capacitance per unit area than adielectric varactor, thereby requiring a larger die area to implement agiven capacitance.

Generally, a varactor may be considered a two-port device, e.g., havinga single input terminal and a single output terminal. As such, varactorsmay be prone to self-modulation distortion resulting from applied radiofrequency (RF) voltages. This self-modulation distortion may introducenonlinearity into a circuit using the varactors. To reduce thisnonlinearity to acceptable levels, a number of individual varactors maybe coupled in series to divide the RF voltage across them. If the numberof varactors in the series is n, then the die area on the circuit boardrequired to realize a desired net capacitance may be increased by afactor of n2 if the varactors are co-planar to one another. If arelatively large number of varactors are used, then this circuit maymake the required die area prohibitively large for use in moderndevices.

In some cases such as oscillator or voltage-controlled oscillator (VCO)circuits in a mobile device, a high quality factor of greater thanapproximately 50 for the varactor may be desirable for increasingefficiency or reducing battery drain of the mobile device. Generally,the quality factor may be considered to be a measurement of thereactance of the varactor (e.g., the impedance presented to an RF signalpropagating through the varactor) compared to the resistance of thevaractor. However, if the collector or sub-collector of the varactor hasa relatively high resistivity, such a quality factor may be difficult toachieve.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements and in which:

FIG. 1a illustrates an example compound varactor circuit, with aparallel resistive bias network, in accordance with various embodiments.

FIG. 1b illustrates another example of a compound varactor circuit witha resistive bias network, in accordance with various embodiments.

FIG. 1c illustrates another example of a compound varactor circuit witha resistive bias network, in accordance with various embodiments.

FIG. 2a illustrates an example circuit diagram of a series-connectedpair of reverse-connected varactors, in accordance with variousembodiments.

FIG. 2b illustrates an example overhead view of a series-connected pairof equal reverse-connected varactors that may be used in the circuit ofFIG. 2a , in accordance with various embodiments.

FIG. 2c illustrates an example overhead view of a non-equalseries-connected pair of equal reverse-connected varactors that may beused in the circuit of FIG. 2a , in accordance with various embodiments.

FIG. 3 illustrates a cut-away view of the series-connected pair of equalreverse-connected varactors of FIG. 2b , in accordance with variousembodiments.

FIG. 4a illustrates an equivalent-circuit model that may be used tostudy varactor Q dependence on sub-collector resistivity, in accordancewith various embodiments.

FIG. 4b illustrates a varactor with a square footprint and associatedquality factor versus frequency dependence, in accordance with variousembodiments.

FIG. 4c illustrates a varactor with a wide aspect ratio footprint andassociated quality factor versus frequency dependence, in accordancewith various embodiments.

FIG. 5a illustrates a simplified example of a circuit that includes aseries-connected pair of reverse-connected varactors, in accordance withvarious embodiments.

FIG. 5b illustrates a series connection of two series-connected pairs ofequal reverse-connected varactors that may be used in the circuit ofFIG. 5a , in accordance with various embodiments.

FIG. 5c illustrates a series connection of two series-connected pairs ofnon-equal reverse-connected varactors that may be used in the circuit ofFIG. 5a , in accordance with various embodiments.

FIG. 5d illustrates an alternative series connection of twoseries-connected pairs of equal reverse-connected varactors that may beused in the circuit of FIG. 5a , in accordance with various embodiments.

FIG. 5e illustrates an alternative series connection of twoseries-connected pairs of non-equal reverse-connected varactors that maybe used in the circuit of FIG. 5a , in accordance with variousembodiments.

FIG. 6a illustrates a simplified example of a circuit that includes aplurality of series-connected pairs of reverse-connected varactors, inaccordance with various embodiments.

FIG. 6b illustrates an example a plurality of series-connected pairs ofnon-equal reverse-connected varactors that may be used in the circuit ofFIG. 6a , in accordance with various embodiments.

FIG. 6c illustrates an alternative example of a plurality ofseries-connected pairs of non-equal reverse-connected varactors that maybe used in the circuit of FIG. 6a , in accordance with variousembodiments.

FIG. 7 illustrates a process for constructing a compound varactor, inaccordance with various embodiments.

FIG. 8 illustrates an alternative overhead view of a compound varactor,in accordance with various embodiments.

FIG. 9 illustrates an alternative overhead view of a system thatincludes a plurality of compound varactors, in accordance with variousembodiments.

FIG. 10 illustrates an alternative overhead view of a system thatincludes a plurality of compound varactors, in accordance with variousembodiments.

FIG. 11 is a block diagram of an exemplary wireless communicationdevice, in accordance with various embodiments.

DETAILED DESCRIPTION

Embodiments include apparatuses and methods related to a compoundvaractor. Generally, a compound varactor may refer to a compactconfiguration of two varactors. A first varactor in the compoundvaractor may include a collector layer and a first base layer that isarranged in a first plurality of parallel fingers. A second varactor inthe compound varactor may include a second base layer arranged in asecond plurality of parallel fingers, and the base layer may be coupledwith the collector layer. In embodiments, the fingers of the base layersof the first varactor and the second varactor may be interleaved withone another.

In some embodiments, the fingers of the first varactor may have a firstwidth, and the fingers of the second varactor may have a second widththat may be the same as or different than the first width. In someembodiments, a plurality of compound varactors may be coupled with oneanother in series or parallel.

Various aspects of the illustrative embodiments will be described usingterms commonly employed by those skilled in the art to convey thesubstance of their work to others skilled in the art. However, it willbe apparent to those skilled in the art that alternate embodiments maybe practiced with only some of the described aspects. For purposes ofexplanation, specific devices and configurations are set forth in orderto provide a thorough understanding of the illustrative embodiments.However, it will be apparent to one skilled in the art that alternateembodiments may be practiced without the specific details. In otherinstances, well-known features are omitted or simplified in order not toobscure the illustrative embodiments.

Further, various operations will be described as multiple discreteoperations, in turn, in a manner that is most helpful in understandingthe present disclosure; however, the order of description should not beconstrued as to imply that these operations are necessarily orderdependent. In particular, these operations need not be performed in theorder of presentation.

The phrase “in one embodiment” is used repeatedly. The phrase generallydoes not refer to the same embodiment; however, it may. The terms“comprising,” “having,” and “including” are synonymous, unless thecontext dictates otherwise.

In providing some clarifying context to language that may be used inconnection with various embodiments, the phrases “A/B” and “A and/or B”mean (A), (B), or (A and B); and the phrase “A, B, and/or C” means (A),(B), (C), (A and B), (A and C), (B and C) or (A, B and C).

The term “coupled with,” along with its derivatives, may be used herein.“Coupled” may mean one or more of the following. “Coupled” may mean thattwo or more elements are in direct physical or electrical contact.However, “coupled” may also mean that two or more elements indirectlycontact each other, but yet still cooperate or interact with each other,and may mean that one or more other elements are coupled or connectedbetween the elements that are said to be coupled with each other.

Various figures may depict various vertical stacks of layers which maybe epitaxially deposited. The sizes, widths, or heights of the variouslayers are not drawn to scale, and should not be assumed to be limitedto being identical to, or different from, one another unless explicitlyindicated to be so in the description below.

As noted above, in certain applications a varactor or varactors with ahigh quality factor may be desirable to maximize signal quality whileminimizing battery drain. It has been observed in legacy varactors thatby increasing the width of a varactor while reducing the length, thesignal quality may be increased. However, there may be limits to theamount in which the width or length of the varactor may be altered basedon the amount of space available in a given system or on a given circuitboard. Also, the propagation of a radio frequency (RF) signal throughthe varactor may cause the varactor to self-modulate, which may beundesirable.

FIG. 1a illustrates an example compound varactor circuit 100 with aparallel resistive bias network, in accordance with various embodiments.The circuit 100 may include a plurality of varactors such as varactors105 a, 105 b, 105 c, 105 d, 105 e, or 105 f (collectively varactors 105)generally positioned between an input terminal 110 and an outputterminal 115. In embodiments, each of varactors 105 may be identical toone another, while in other embodiments one or more of the varactors 105may be different from another of the varactors 105. Examples ofdifferences between varactors 105 are discussed in greater detail belowwith respect to FIG. 1b and elsewhere. In some embodiments, the inputterminal 110 may be configured to receive a radio frequency (RF) signalthat may then propagate through the circuit 100 to the output terminal115 (or vice versa). In some embodiments, one or more of the varactors105 may be connected in parallel with the input terminal 110 and theoutput terminal 115, in which case the RF signal may not propagatethrough the varactor to the output terminal 115. In some cases, thecircuit 100 may be used in shunt across an RF line in which case theoutput terminal 115 may be coupled with ground.

In some embodiments, each of the varactors 105 may have a “front” sideand a “back” side. FIG. 1a depicts the front side 107 and back side 109of varactor 105 a. In embodiments the front side 107 of varactor 105 amay be referred to as the “cathode” of varactor 105 a, and the back side109 of varactor 105 a may be referred to as the “anode” of varactor 105a. In FIG. 1, each of the varactors 105 may have a front side and a backside (or cathode and anode), though specific designators in FIG. 1 areomitted for each varactor for the sake of clarity.

In some embodiments, two or more of the varactors 105 may be coupledwith one another in a back-to-back configuration. Specifically, theanodes of the varactors may be coupled directly to one another. Forexample, varactors 105 b and 105 c may be considered to be in aback-to-back configuration as shown in FIG. 1a . In other embodiments,the varactors 105 may be coupled with one another in a front-to-frontconfiguration as shown in FIG. 1a . Specifically, the cathodes of thevaractors may be coupled directly to one another. For example, varactors105 a and 105 b may be considered to be in a front-to-frontconfiguration as shown in FIG. 1.

In embodiments, the back sides of one or more of the varactors 105 maybe coupled with ground 120. Additionally, the front sides of one or moreof the varactors 105 may be coupled with a DC power source 125. The DCpower source 125 may be configured to provide a positive control voltage(VcTRL) to reverse bias the varactors 105. In some embodiments, VcTRLmay be between approximately 2 Volts (V) and approximately 18 V, whilein other embodiments VcrnL may be between approximately −1.2 V andapproximately 3 V. In other embodiments (not shown), the front sides ofthe varactors 105 may be coupled with ground 120, and the back sides ofthe varactors 105 may be coupled with a DC power source 125. In thoseembodiments, the DC power source 125 may be configured to provide anegative VcrnL to reverse bias the varactors 105. Other more complicatedcircuits may be envisioned having multiple DC power sources that mayeach supply different or similar positive or negative voltages, ormultiple ground connections.

In embodiments one or more resistors such as resistors 135 a, 135 b, 135c, 135 d, 135 e, 135 g, and 135 f (collectively resistors 135) may bepositioned between the varactors 105 and the ground 120 or the DC powersource 125. In some embodiments, the resistance of each of the resistors135 may be equal while in other embodiments certain of the resistors 135such as outer resistors 135 f or 135 g may be greater than others of theresistors 135. For example, in embodiments, the resistance of resistors135 f and/or 135 g may be approximately 60 kilo-ohms (kO), while inother embodiments the resistance of resistors 135 f and/or 135 g may bebetween approximately 20 kO and approximately 60 kO. Similarly, in someembodiments the resistance of resistors 135 a, 135 b, 135 c, 135 d, or135 e may be approximately 30 kO, while in other embodiments theresistance of resistors 135 a, 135 b, 135 c, 135 d, or 135 e may bebetween approximately 10 kO and approximately 30 kO.

As shown above, the circuit 100 may include a number of varactors 105and resistors 135. Although only six varactors 105 and five resistors135 are shown in FIG. 1, in other embodiments the circuit 100 mayinclude a greater or lesser number of varactors 105 or resistors 135. Insome embodiments, it may be desirable for the circuit to include atleast the resistors 135 a and 135 e. In some embodiments, inductors mayalso be used in place of, or in combination with, the resistors 135. Asdiscussed above, as the number of varactors 105 in the compound varactor100 increases, the area that the compound varactor 100 requires on a diemay increase exponentially if all of the varactors 105 are co-planar toone another.

FIG. 1b illustrates an example of a compound varactor circuit with analternative resistive bias network. Specifically, the circuit 101 mayinclude a series tree-type bias network in place of the parallelresistive bias network of FIG. 1a . In embodiments, circuit 101 mayinclude the input terminal 110, DC power source 125, ground 120, andterminal 115, and resistors 135 as described above with respect to FIG.1a . Circuit 101 may further include varactors 145 and 150, which may besimilar to varactors 105 of FIG. 1a . In embodiments, each of varactors145 and 150 may be similar to one another, while in other embodimentsthe varactors 145 may be similar to one another, but different fromvaractors 150, as described below.

Generally, circuits 100 and 101 may be more desirably used in shuntacross the RF line, with output terminal 115 coupled with ground. FIG.1c illustrates an example of a compound varactor circuit 102 with analternative resistive bias network. Specifically, circuit 102 mayinclude a series tree-type resistive bias network with a symmetric biasfeed that may be more suitable for a varactor to be used in series withthe RF line. Specifically, circuit 102 may include the input terminal110, DC power source 125, ground 120, and terminal 115, and resistors135 as described above with respect to FIG. 1b . Circuit 102 may furtherinclude varactors 145 and 150, which may be similar to varactors 145 and150 of FIG. 1b . In embodiments, each of varactors 145 and 150 may besimilar to one another, while in other embodiments the varactors 145 maybe similar to one another, but different from varactors 150, asdescribed below.

Generally, any of circuits 100, 101, and 102 may be used if DC powersource 125 is coupled to the back sides of varactors 105, 145, and 150and configured to provide a negative bias control voltage, as describedabove.

Typically, in a varactor such as one of varactors 105, 145, or 150, thetop region of the varactor (e.g., base or anode) may be coupled with ametalized layer of Aluminum (Al), Copper (Cu), Gold (Au), or some othermetal or alloy with a relatively low resistivity. This metalized layerof the varactor may be used as an electrode that has a very lowresistance, which results in very low loss of RF energy flowing throughthe varactor.

In contrast, the bottom region of the varactor (e.g., collector orcathode) may be coupled with a sub-collector doped region. Typically,the resistivity of the collector and/or sub-collector may have aresistivity that is significantly higher than that of the metalizedlayer. For example, in some embodiments the resistivity of the collectorand/or sub-collector may be an order of magnitude greater than theresistivity of the metalized layer.

The quality factor Q of a varactor may be defined in terms of theangular frequency ω (2π*f, where f may be the frequency of the RF signalpassing through the varactor), C (the varactor capacitance), and R (aresistive component). Specifically, the quality factor Q may be

$Q\; \alpha {\frac{1}{\omega \; C\; R}.}$

Generally, R may be an equivalent series resistance value that mayaccount for dissipative losses in the varactor. In high Q varactors,this resistive component may arise primarily based on resistance in thebottom electrode of the varactor, e.g., the collector/sub-collector.

To model the losses associated with the high resistivity of thecollector/sub-collector region, an equivalent circuit model 400 such asthat illustrated in FIG. 4a may be used. Specifically, the model 400 mayinclude a plurality of capacitors C and resistors R. Using the model400, FIG. 4b shows the predicted quality factor dependence overfrequency of a varactor 405 with a square footprint.

The model prediction for the quality factor dependence over frequency ofa varactor 410 with an equivalent footprint, but having a wide aspectratio, is shown for comparison in FIG. 4c . It can be seen that avaractor layout with a wider form factor may have a substantiallyimproved quality factor Q. This improved quality factor Q may be due tothe shorter propagation distance in the collector/sub-collector betweenports 1 and 2. In general, if the varactor area and capacitance is keptconstant, the quality factor Q of the varactor may have the followingdependence on the width of the varactor: QαW². Therefore, making thevaractor as wide as possible in a direction normal to the input andoutput ports may maximize the quality factor Q of the varactor. However,in practice there may be limits on how wide the varactor can be made.

FIG. 2a illustrates an example circuit for a compound varactor 290 thatincludes a pair of reverse-biased varactors having a series connectionwith an increased quality factor Q. Specifically, the compound varactor290 may include a first varactor 291 and a second varactor 292 in afront-to-front configuration. Additionally, in some embodiments circuit200 may include one or more resistors such as resistors 135, a groundconnection such as ground 120, a DC power source such as power source125, an input terminal such as input terminal 110, and/or an outputterminal such as output terminal 115 as shown in FIG. 1 a.

FIG. 2b illustrates an overhead view of a compound varactor 200 that maycorrespond to the compound varactor 290 and exhibit an increased qualityfactor Q, in accordance with various embodiments. As will be recognized,the compound varactor 200 may include a pair of front-to-front varactorsthat both include a plurality of electrode fingers. As noted above, thecollector/sub-collector of the compound varactor 200 may have arelatively high resistance compared to the resistance of the electrodefingers. By implementing two front-to-front varactors with a pluralityof electrode fingers, the distance that the signal is required to travelthrough the collector/sub-collector layer may be reduced or minimized,which may result in a significant reduction in resistance experienced byan RF signal traveling through the compound varactor 200. This reductionin resistance may result in an increase in the quality factor Q.

An additional advantage of the compound varactor 200 may be that incertain legacy embodiments only a single varactor may be implementedsuch that electrode fingers are used for a first port of the varactor,and the second port of the varactor may be coupled directly with thecollector/sub-collector layer through one or more vias. This legacydesign may exhibit a significantly decreased capacitance per unit area,and an increased quality factor Q, because the second port that iscoupled directly with the collector/sub-collector layer may not resultin a measurable capacitance because the corresponding base layer in thatregion may be missing. In other words, the space where the second portis coupled directly with the collector/sub-collector layer may not havea significant electrical effect on the RF signal traveling through thevaractor. By contrast, in embodiments herein, the base layer may becontinuous under all of the electrode fingers. As such, the compoundvaractor 200 may experience only a minimal reduction in the capacitanceper unit area based on the fingers of the electrodes, resulting in asignificant advantage in terms of both size and cost of the invention.This reduction in capacitance per unit area may be a result of the smallbut necessary gaps between the electrodes.

In embodiments, the compound varactor 200 may include two separatevaractors 205 and 210 (that may respectively correspond to varactors 291and 292), and have a Length and a Width as designated in FIG. 2b . Inembodiments, varactors 205 and 210 may be similar to one of varactors105. Specifically, the first varactor 205 may include a collector layer215. The collector layer 215 may be composed of or include asemiconductor material such as gallium arsenide, silicon, germanium,aluminum phosphide, aluminum arsenide, indium phosphide, galliumnitride, combinations or alloys thereof, or some other semiconductor. Insome embodiments, the collector layer 215 may be doped or heavily dopedwith one or more impurities such as carbon, zinc, beryllium, or someother dopant. In embodiments, the collector layer 215 may be doped withan “n+” dopant.

The varactor 205 may include a base layer 220 directly coupled with thecollector layer 215. The base layer 220 may be composed of or includeone or more of the semiconductor materials discussed above, but the baselayer 220 may be doped with a “p+” dopant. The varactor 205 may furtherinclude a first metal layer 225 directly coupled with the base layer220. The first metal layer 225 may be composed of or include titanium,platinum, gold, zinc, nickel, beryllium, or combinations or alloysthereof. The varactor 205 may further include a second metal layer 230directly coupled with the first metal layer 225. The second metal layer230 may be composed of or include one or more of the same materialslisted above as the first metal layer 225. In some embodiments, thefirst metal layer 225 and second metal layer 230 may be composed ofidentical materials, while in other embodiments the first metal layer225 and second metal layer 230 may be composed of different materials.In some embodiments, vias 235 may communicatively connect two or more ofthe base layer 220 or the metal layers 225 and 230. In some embodiments,one or both of the metal layers 225 or 230 may not be included incompound varactor 200.

The second varactor 210 may be similar to the first varactor 205, andinclude the collector layer 215, a base layer 240, a first metal layer245, and a second metal layer 250, which may be similar to base layer220, first metal layer 225, and second metal layer 230, respectively. Inembodiments, vias 235 may electrically couple one or more of base layer240, metal layer 245, and metal layer 250, as described above. In someembodiments, the collector layer 215 may include a sub-collector layer(not shown) that is directly coupled with the collector layer 215 on aside of the collector layer 215 directly opposite the base layers 220and 240. It will be understood that although each of the layers isdepicted in FIG. 2 as smaller than or inside of one or more otherlayers, such depiction is done for the ease of understanding therelative positioning of the layers, and in some embodiments differentlayers such as the base layer 220 and metal layer 225 may haveapproximately similar or identical lengths or widths. In someembodiments, the collector layer 215 may be coupled with a bias input280 that may be coupled with and configured to receive a voltage biasfrom, for example, DC power source 125.

In some embodiments, one or more of the base layer 220 or metal layers225 or 230 of the first varactor 205 may be constructed as a pluralityof generally parallel fingers 260 that define one or more lateralcavities or spaces 265. Similarly, one or more of the base layer 240 ormetal layers 245 or 250 of the second varactor 210 may be constructed asgenerally parallel fingers 270 that define a plurality of spaces 275.Fingers 260 and 270 may be the “electrode fingers” discussed above. Asshown, fingers 270 may be positioned in spaces 265, and fingers 260 maybe positioned in spaces 275 such that the fingers 260 and 270 areconsidered to be interspersed or interleaved with one another.

In embodiments, the collector layer 215 may act as the cathode ofvaractors 205 and 210, while the base layers 220 and 240 may act as theanodes of varactors 205 and 210, respectively. That is, varactor 205 maybe communicatively coupled with an input terminal such as input terminal110 that is configured to provide an RF signal. The RF signal maypropagate through the layers of varactor 205 to the collector layer 215.The RF signal may then propagate through the collector layer 215 andback up through the various layers of varactor 210, which in turn iscommunicatively coupled with an RF output terminal such as outputterminal 115.

In the example compound varactor 200, the signal may propagate along thelength of the compound varactor 200. Specifically, as compound varactor200 is depicted in FIG. 2b , the RF signal may propagate from the top tothe bottom of FIG. 2b or vice-versa. As noted above, the collector layer215 may have a relatively high resistance, for example, on the order of6 Ohms per square. By contrast, the resistance of the base layers ormetal layers of varactors 205 or 210 may be very low. Therefore, it maybe more desirable for a signal to propagate primarily through the baselayers 220 and 240 or metal layers 225, 230, 245, and 250 of varactors205 or 210 such that the distance that the signal has to propagatethrough collector layer 215 is minimized. By constructing varactors 205and 210 to include a plurality of fingers 260 and 270, the signal mayonly travel through the collector layer 215 a short distance to one offingers 270, or from one of fingers 260.

In some embodiments, improved performance may be realized by using acombination of different valued varactors in a compound varactor orplurality of compound varactors in series and/or parallel with oneanother. FIG. 2c illustrates an example overhead view of a non-equalseries-connected pair of equal reverse-connected varactors in a compoundvaractor 201. The compound varactor 201 may have a relatively high Qcompared to, for example, compound varactor 200.

The compound varactor 201 may include first and second varactors 206 and211, which may be similar to first and second varactors 205 and 210.Specifically, varactor 206 may include a collector layer 216, a baselayer 221, a first metal layer 226, and a second metal layer 231, whichmay be respectively similar to collector layer 215, base layer 220,metal layer 225, and metal layer 230, respectively. Varactor 211 mayinclude the collector layer 216, base layer 241, metal layer 246, andmetal layer 251, which may be respectively similar to base layer 240,metal layer 245, and metal layer 250. In embodiments, varactors 206 and211 may include vias similar to vias 235, and a bias input that may besimilar to bias input 280, both of which are not shown in FIG. 2c forthe sake of clarity. In embodiments, varactor 206 may include fingers261, and varactor 211 may include fingers 271. However, as can be seenin FIG. 2c , in embodiments fingers 261 may be significantly narrowerthan fingers 271. It will be understood that in other embodimentsfingers 271 may be significantly narrower than fingers 261.

In compound varactor 201, the difference in widths of fingers 261 and271 may result in an unequal capacitance between varactors 206 and 211.It should be noted that the number, width, and length of the fingers 261and 271 may not be uniquely constrained, but may be flexible parametersthat may be optimized to best achieve desired performance parameters. InFIG. 2c , for example, each of the arrays is depicted as having threefingers. However, the capacitance of each of the varactors 206 and 211may be solely dependent upon the total area of the array of fingers.Thus, the two series-connected capacitances may be equally-well achievedwith five fingers, seven fingers, or some other number of fingers ineach of the arrays in other embodiments. In the case where there aremore fingers in a given varactor, the fingers may be narrower than shownin either compound varactors 200 or 201. Alternatively, in cases wherethere are less fingers in a given varactor, the fingers may be widerthan shown in either compound varactors 200 or 201. In theseembodiments, the wider fingers may result in lower series resistance inthe fingers, which may benefit performance. However, the widerelectrodes may also mean an increase in the mean propagation distance inthe collector/sub-collector, which in turn may result in an effectiveincrease in the collector resistance. This increased resistance mayresult in a negative impact on the quality factor Q of the device. Thus,the optimum device layout may be a compromise between loss mechanisms inthe varactor, as described above. Factors that may change based on thiscompromise may include the materials used in the compound varactor, thelevel of dopant of the collector/sub-collector layer, the length orwidth of the fingers, the overall size of the compound varactor, orother factors.

FIG. 3 illustrates an example side view of a compound varactor 300 suchas compound varactor 200, taken along line A-A of FIG. 2b . It will beunderstood that the compound varactor 300 of FIG. 3 is intended as anexample to show the relative positions of certain elements of FIG. 2balong the z-axis. As such, relative heights, lengths, or widths ofelements of FIG. 3 should not be considered as definitive unlessexplicitly defined as such below.

In embodiments, the compound varactor 300 may include two varactors 305and 310, which may be similar to compound varactors 205 and 210. Inembodiments, the varactors 305 and 310 may both include a collectorlayer 315, which may be similar to collector layer 215 of FIG. 2b .Varactor 305 may include a base layer 320 and metal layers 325 and 330,which may be respectively similar to base layer 220 and metal layers 225and 230 of FIG. 2b . In embodiments, varactor 305 may further includeone or more vias 335, which may be similar to vias 235 and configured tocommunicatively couple one or more of the base layer 320 or metal layers225 or 230 to one another.

Similarly to varactor 305, varactor 310 may include a base layer 340 andmetal layer 345 and 350, which may be respectively similar to base layer240 and metal layers 245 and 250. In some embodiments, the compoundvaractor 300 may include a sub-collector layer 355 coupled with thecollector layer 315, which may be similar to the sub-collector layerthat is described above, but not shown, with respect to compoundvaractor 200.

In some cases, as illustrated in FIG. 1a, 1b , or 1 c, stacking of morethan two varactor diodes may be desirable to meet linearity requirementsof a circuit or apparatus using compound varactors. In some cases, morethan one pair of inerdigitated varactors may readily be stacked inseries to achieve multi-varactor configurations such as those shown inFIG. 1a, 1b , or 1 c.

FIG. 5a shows a simple circuit diagram of a circuit 500 that includesfour varactors 505, 510, 515, and 520, which may be similar to varactors105, 145, or 150. Specifically, varactors 505 and 510 may be a firstcompound varactor that includes an interdigitated varactor pair, andvaractors 515 and 520 may be a second compound varactor that includes aninterdigitated varactor pair. In some embodiments, varactors 505, 510,515, and 520 may be identical to one another. In other embodiments, oneor more of varactors 505, 510, 515, and 520 may be different fromanother of the varactors. For example, in some embodiments varactor 505may have relatively narrow fingers such as those shown with respect tovaractor 206 in FIG. 2c , while varactor 510 may have relatively widefingers such as those shown with respect to varactor 211 in FIG. 2c (orvice-versa). Similarly, if varactor 510 has relatively wide fingers,then varactor 515 may have relatively wide fingers and varactor 520 mayhave relatively narrow fingers (or vice-versa). In some embodiments, ifvaractor 510 has relatively wide fingers, then varactor 515 may haverelatively narrow fingers and varactor 520 may have relatively widefingers.

Additionally, in some embodiments circuit 500 may include one or moreresistors such as resistors 135, a ground connection such as ground 120,a DC power source such as power source 125, an input terminal such asinput terminal 110, and/or an output terminal such as output terminal115.

FIG. 5b illustrates a series connection of two series-connected pairs ofequal reverse-connected varactors that may be used in the circuit ofFIG. 5a , in accordance with various embodiments. Specifically, FIG. 5billustrates a compound varactor 501 that may include four varactors 506,511, 516, and 521 in a series connection with one another. Varactor 506may include collector layer 531 and base layer 526, which may be similarto collector layer 215 and base layer 220 of FIG. 2b . Varactor 511 mayinclude collector layer 531, and base layer 536, which may be similar tobase layer 240 of FIG. 2b . Varactor 516 may include collector layer541, which may also be similar to collector layer 215, and base layer536. Finally, varactor 521 may include collector layer 541 and baselayer 546, which may be similar to base layer 240 of FIG. 2b . Inembodiments, one or more of varactors 506, 511, 516, and 521 may includeone or more metal layers, a sub-collector layer, or vias, which are notillustrated in FIG. 5b for the sake of clarity. In operation, a signalmay flow from varactor 506 through the compound varactor 501 and exitthe compound varactor 501 at varactor 521 (or vice-versa). Although eachof the varactors in compound varactor 501 are shown as having only threefingers, in other embodiments the varactors in compound varactor 501 mayhave more or less fingers.

FIG. 5c illustrates a series connection of two-series-connected pairs ofnon-equal reverse-connected varactors that may be used in the circuit ofFIG. 5a , in accordance with various embodiments. Specifically, FIG. 5cillustrates a compound varactor 502 that may include four varactors 507,512, 517, and 522 in a series connection with one another. Varactor 507may include collector layer 532 and base layer 527, which may be similarto collector layer 216 and base layer 221 of FIG. 2c . Varactor 512 mayinclude collector layer 532, and base layer 537, which may be similar tobase layer 241 of FIG. 2c . Varactor 517 may include collector layer542, which may also be similar to collector layer 216, and base layer537. Finally, varactor 522 may include collector layer 542 and baselayer 547, which may be similar to base layer 241 of FIG. 2c . Inembodiments, one or more of varactors 507, 512, 517, and 522 may includeone or more metal layers, a sub-collector layer, or vias, which are notillustrated in FIG. 5c for the sake of clarity. In operation, a signalmay flow from varactor 507 through the compound varactor 502 and exitthe compound varactor 502 at varactor 522 (or vice-versa). Although thefingers of varactors 507 and 522 are shown as relatively narrow and thefingers of varactors 512 and 517 are shown as relatively wide, in otherembodiments the fingers of varactors 507 and 522 may be relatively wide,and the fingers of varactors 512 and 517 may be relatively narrow.Although each of the varactors in compound varactor 502 are shown ashaving only three fingers, in other embodiments the varactors incompound varactor 502 may have more or less fingers.

While functional, the simple stacking architectures of compoundvaractors 501 and 502 may experience energy from a signal flowingthrough the compound varactors 501 and 502 flowing transversely alongthe horizontal busbars of the varactors 501 and 502. This energy mayflow transversely along the horizontal busbars because of the verticaldiscontinuities of base layer 537. Specifically, a signal flowingvertically (as seen in the FIG. 5b or 5 c) may flow into varactor 511 or512, but then have to flow horizontally through base layer 536 or 537 tothe fingers of varactors 516 or 517.

FIG. 5d illustrates an alternative series connection oftwo-series-connected pairs of equal reverse-connected varactors that maybe used in the circuit of FIG. 5a , in accordance with variousembodiments. Specifically, FIG. 5d illustrates a compound varactor 503that may include four varactors 508, 513, 518, and 523 in a seriesconnection with one another. Varactor 508 may include collector layer533 and base layer 528, which may be similar to collector layer 215 andbase layer 220 of FIG. 2b . Varactor 513 may include collector layer533, and base layer 538, which may be similar to base layer 240 of FIG.2b . Varactor 518 may include collector layer 543, which may also besimilar to collector layer 215, and base layer 538. Finally, varactor523 may include collector layer 543 and base layer 548, which may besimilar to base layer 240 of FIG. 2b . In embodiments, one or more ofvaractors 508, 513, 518, and 523 may include one or more metal layers, asub-collector layer, or vias, which are not illustrated in FIG. 5d forthe sake of clarity. In operation, a signal may flow from varactor 508through the compound varactor 503 and exit the compound varactor 503 atvaractor 523 (or vice-versa). Although each of the varactors in compoundvaractor 503 are shown as having only three fingers, in otherembodiments the varactors in compound varactor 503 may have more or lessfingers.

In embodiments, the signal may experience less loss in compound varactor503 than, for example, compound varactor 501 because the fingers ofvaractors 513 and 518 may be vertically aligned with one another, asshown in FIG. 5d . Therefore, if the signal is flowing verticallythrough compound varactor 503, then the signal may not have to flowtransversely through base layer 538 to move from the fingers of varactor513 to the fingers of varactor 518.

FIG. 5e illustrates an alternative series connection oftwo-series-connected pairs of non-equal reverse-connected varactors thatmay be used in the circuit of FIG. 5a , in accordance with variousembodiments. Specifically, FIG. 5e illustrates a compound varactor 503that may include four varactors 509, 514, 519, and 524 in a seriesconnection with one another. Varactor 509 may include collector layer534 and base layer 529, which may be similar to collector layer 216 andbase layer 221 of FIG. 2c . Varactor 514 may include collector layer534, and base layer 539, which may be similar to base layer 241 of FIG.2c . Varactor 519 may include collector layer 544, which may also besimilar to collector layer 216, and base layer 539. Finally, varactor524 may include collector layer 544 and base layer 549, which may besimilar to base layer 241 of FIG. 2c . In embodiments, one or more ofvaractors 509, 514, 519, and 524 may include one or more metal layers, asub-collector layer, or vias, which are not illustrated in FIG. 5e forthe sake of clarity. In operation, a signal may flow from varactor 509through the compound varactor 504 and exit the compound varactor 504 atvaractor 524 (or vice-versa). Although the fingers of varactors 509 and524 are shown as relatively narrow and the fingers of varactors 514 and519 are shown as relatively wide, in other embodiments the fingers ofvaractors 509 and 524 may be relatively wide, and the fingers ofvaractors 514 and 519 may be relatively narrow. Although each of thevaractors in compound varactor 504 are shown as having only threefingers, in other embodiments the varactors in compound varactor 504 mayhave more or less fingers.

Similarly to compound varactor 503, a signal flowing through compoundvaractor 504 may experience less loss in compound varactor 504 than, forexample, compound varactor 502 because the fingers of varactors 514 and519 may be vertically aligned with one another, as shown in FIG. 5e .Therefore, if the signal is flowing vertically through compound varactor504, then the signal may not have to flow transversely through baselayer 539 to move from the fingers of varactor 514 to the fingers ofvaractors 519.

In some embodiments, an anti-parallel pair of stacked diodes may bebeneficial as shown in FIG. 6a . Specifically, as mentioned above, insome uses dual interconnected varactor stacks may be advantageous. Sucha circuit could be desirable, for example, because in some embodimentsparallel asymmetric varactor stacks with non-equal capacitance ratiosmay increase linearity. Additionally, an interdigitatedreverse-connected varactor pair configuration may be particularlyspace-efficient in realizing such dual stacked pairs.

FIG. 6a depicts a high-level circuit diagram of a circuit 600 thatincludes two sets of series-stacked varactors. Specifically, the circuit600 may include varactors 605, 610, 615, 620, 625, 630, 635, and 640,which may be similar to varactors 105, 145, and/or 150. In embodiments,certain of the varactors such as varactors 605 and 610 may befront-to-front with one another, while others of the varactors such asvaractors 610 and 615 may be back-to-back with one another. Inembodiments, the stacks may be connected to one another viainterconnects such as interconnects 645, 650, and 655.

In some embodiments, each of the varactors 605, 610, 615, 620, 625, 630,635, and 640 may be similar to one another, for example having similarfinger width or constructed of the same materials. In other embodiments,at least one of varactors 605, 610, 615, 620, 625, 630, 635, and 640 maybe different from another one of the varactors, for example having adifferent finger width or being constructed of a different material fromthe other varactor. In some embodiments, varactors 605, 620, 630, and635 may be similar to one another, but different from varactors 610,615, 625, and 640 (which may be similar to one another).

Although not shown for the sake of simplicity, in some embodimentscircuit 600 may include more or fewer varactors than are shown in FIG.6a . Additionally, in some embodiments circuit 600 may include one ormore resistors such as resistors 135, a ground connection such as ground120, a DC power source such as power source 125, an input terminal suchas input terminal 110, and/or an output terminal such as output terminal115.

FIG. 6b illustrates an example a plurality of series-connected pairs ofnon-equal reverse-connected varactors that may be used in the circuit ofFIG. 6a , in accordance with various embodiments. Specifically, FIG. 6bdepicts a compound varactor 602 that includes two varactor stacks 662and 672 of varactors that are in series with one another as shown inFIG. 6a . In embodiments, varactor stacks 662 and 672 may be in parallelwith one another.

Specifically, stack 662 may include varactors 607, 612, 617, and 622,which may be similar to varactors 509, 514, 519, and 524, respectively.Similarly, stack 672 may include varactors 627, 632, 637, and 642, whichmay also be similar to varactors 509, 514, 519, and 524, respectively.In embodiments, the base layers of stacks 662 and 672 may be coupledwith one another as shown in FIG. 6b . Varactors 607 and 612 may sharecollector layer 646. Varactors 617 and 622 may share collector layer651. Varactors 627 and 635 may share collector layer 656. Varactors 637and 642 may share collector layer 661.

As shown, in some embodiments the configurations of the stacks 662 and672 may be different. For example, the “inner” varactors 612 and 617 ofstack 662 may have relatively wide fingers, while the “outer” varactors607 and 622 of stack 662 may have relatively narrow fingers. Bycontrast, in stack 672 the “inner” varactors 632 and 637 may haverelatively narrow fingers while the “outer” varactors 627 and 642 mayhave relatively wide fingers. In other embodiments, the widths of the“inner” fingers of the fingers of stack 662 and the “outer” fingers ofstack 672 may be relatively narrow while the widths of the “inner”fingers of the stack 672 and the “outer” fingers of the stack 662 may berelatively wide. As described herein, “inner” and “outer” are onlyintended as descriptive elements to identify the different fingers ofthe different varactors in FIG. 6b , and are not intended as limiting ordefinitional elements.

In some embodiments, the collector layers of the compound varactors maybe coupled with one or more DC power sources such as DC power source 125that may be configured to provide a DC voltage bias. Specifically,collector layers 646, 651, 656, and 661 may be coupled with a DC powersource via interconnects 645, 655, 650, and 660, respectively. In someembodiments, one or more resistors such as resistors 135 may bepositioned between one or more of the collector layers 646, 651, 656,and 661 and the DC power source. In embodiments, the various varactorsof compound varactor 602 may include one or more of vias, metal layers,or sub-collector layers, which are not shown in FIG. 6b for the sake ofclarity. Although the varactors of compound varactor 602 are shown withthree fingers each, in other embodiments the varactors may have agreater or lesser number of fingers. Similarly, it can be seen that thefingers of the base layers of varactors 612, 617, 632, and 637 are widerthan the fingers of the base layers of varactors 607, 622, 627, and 642.In other embodiments, the fingers of the base layers of varactors 612,617, 632, and 637 may be narrower than the fingers of the base layers ofvaractors 607, 622, 627, and 642.

FIG. 6c illustrates an alternative example of a plurality ofseries-connected pairs of non-equal reverse-connected varactors that maybe used in the circuit of FIG. 6a , in accordance with variousembodiments. Specifically, the dual stack architecture of FIG. 6a or 6 bmay result in significantly increased linearity for a signal propagatingthrough the compound varactor. Non-linear artifacts from the two stacksmay cancel each other via the interconnects between the two stacks.However, if each of the stacks has a wide aspect ratio, which may bedesirable as required for increasing or maximizing the quality factor Qof the compound varactor, then resistance and inductance in thehorizontal connections between the stacks may inhibit the cancellationof the spurious artifacts. To reduce any such degradation inperformance, the left and right stacks may be broken up intosub-sections, or segmented, and interspersed with one another asillustrated in FIG. 6 c.

Specifically, FIG. 6c illustrates a compound varactor 603 that mayconsist of four stacks of series varactors. The stacks may be similar tostacks 662 or 672, but they may be segmented versions of stacks such asstacks 662 or 672. Specifically, stacks 695 and 697 may be a segmentedversion of a stack such as stacks 662 or 672, and stacks 696 and 698 maybe a segmented version of a stack such as stacks 662 or 672.

For example, even though the varactors of stacks 695, 696, 697, or 698are shown as having three fingers each, the number of fingers is shownsimply as an example and is not intended to be determinative. In someembodiments, because stacks 695 and 697 are segmented portions of one ofthe stacks shown in FIG. 6a , the number of fingers of varactors instacks 695 and 697 combined may be equal to the number of fingers ofvaractors in stack 662. In other words, stack 662 may be segmented toform stacks 695 and 697. Similarly, stacks 696 and 698 may be asegmented version of other stacks described herein.

In embodiments, the collector layers (not labeled for the sake ofclarity) of elements of a single segmented stack may be coupled to oneanother. For example, stacks 695 and 697 may be segmented elements of astack such as stack 662, as described above. The collector layers ofstacks 695 and 697 may be coupled together by interconnect 699 a, whichmay be similar to one of interconnects 645, 650, 655, or 660, andfurther coupled with a DC power source as described above with respectto FIG. 6b . Similarly, the collector layers of stacks 696 and 698 maybe coupled together by interconnect 699 b. By coupling the collectorlayers of stacks 695 and 697, or 696 and 698, together, a similar DCvoltage bias may be applied to both segmented elements of a stack.

It may further be seen that the varactors of the various stacks ofcompound varactor 603 may be unequal, that is having different fingerwidths, similarly to compound varactor 602 of FIG. 6b . Additionally,the fingers widths of the varactors of the stacks may not vary in thesame pattern. Specifically, as can be seen the outer varactors of stacks695 and 697 have relatively narrow fingers, while the inner varactors ofstacks 695 and 697 have relatively wider fingers. By contrast, the outervaractors of stacks 696 and 698 have relatively wide fingers, while theinner varactors of stacks 696 and 698 have relatively narrow fingers. Bysegmenting the various stacks of the compound varactor circuit of FIG.6a , and interleaving the segmented stacks, the second-order non-linearcomponents of the RF signal may be reduced or minimized.

FIG. 7 depicts an example process for generating a compound varactorsuch as compound varactor 200. Initially, a collector layer such ascollector layer 215 may be deposited at 700. Next, a base layer such asbase layer 220 may be deposited on the collector layer 215 at 705.Finally, a base layer such as base layer 240 may be deposited on thecollector layer 215 at 710.

As described herein, the deposition of the base layers at 710 and 715may include depositing the base layer to form fingers such as fingers260 or 270. In some embodiments, the base layer may be deposited andthen etched to form the fingers by mechanical, electrical, or chemicaletching. In some embodiments, only a single base layer may be depositedand then etched to form the fingers of the two varactors 205 and 210 inthe compound varactor 200. In some embodiments, additional layers suchas the sub-collector layer or one or more of the metal layers asdescribed above may be deposited and/or etched.

FIG. 8 depicts an alternative embodiment of a compound varactor 800 thatmay include elements that are similar to elements of compound varactor200, and are labeled similarly. For example, compound varactor 800 mayinclude varactors 805 and 810. Varactor 805 may include the collectorlayer 815, one or more base layers 820, one or more metal layers 825,and a second metal layer 830 which may be respectively similar tocollector layer 215, base layer 220, metal layer 225, and metal layer230. In embodiments, vias 835 may electrically connect one or more ofbase layer 820, metal layer 825, and metal layer 830.

Similarly, varactor 810 may include the collector layer 815, one or morebase layers 840, one or more metals layers 845, and a metal layer 850that may be respectively similar to base layer 240, metal layer 245, andmetal layer 250. In embodiments, vias 835 may electrically connect oneor more of base layers 840, metal layer 845, and metal layer 850. As canbe seen in FIG. 8, in embodiments the base layers 820 and 840 may not beformed as fingers, but instead be formed as discrete elements that arearranged generally opposite one another. In embodiments, the base layers820 and 840 may be generally joined by metal layers 825, 830, 845,and/or 850, as shown in FIG. 8.

The collector layer 815 may be coupled with a bias input 880 that may becoupled with and configured to receive a voltage bias from, for example,DC power source 125. In some embodiments, the collector layer 815 mayfurther include or be coupled with a sub-collector layer (not shown).Also, in some embodiments, the different sizes or number of elements maybe different than depicted in FIG. 8. For example, the metal layer 830may have a generally similar width to the total width of the base layer820, rather than being slightly narrower as depicted in FIG. 8.Additionally, in some embodiments the compound varactor 800 may includemore or fewer base layers 820 or 840, or vias 835.

FIG. 9 depicts an alternative embodiment that includes a compoundvaractor 900. Elements of FIG. 9 may be similar to elements of FIG. 2,and numbered similarly. Varactor 905 may include collector layer 915,base layer 920, metal layer 925, and metal layer 930, which may berespectively similar to collector layer 215, base layer 220, metal layer225, and metal layer 230. In embodiments, vias 935, which may be similarto vias 235, may electrically connect one or more of base layer 920,metal layer 925, and metal layer 930. Varactor 910 may include collectorlayer 915 and base layer 940, which may be similar to collector layer215 and base layer 240. Varactor 910 may further include metal layers995 and 990, as described in further detail below. In embodiments, vias935 may electrically connect one or more of base layer 940, metal layer995, and metal layer 990.

Varactor 906 may include collector layer 916 and base layer 921, whichmay be respectively similar to collector layer 215 and base layer 220.Varactor 906 may further include metal layers 990 and 995, as describedin further detail below. In embodiments, vias 935 may electricallyconnect one or more of base layer 921, metal layer 990, and metal layer995. Varactor 911 may include collector layer 916, base layer 941, metallayer 946, and metal layer 950, which may be respectively similar tocollector layer 215, base layer 240, metal layer 245, and metal layer250. In embodiments, vias 935 may electrically connect one or more ofbase layer 941, metal layer 946, and metal layer 950.

In some embodiments the collector layers 915 and 916 may be coupled witha bias input 980 that may be coupled with and configured to receive avoltage bias from, for example, DC power source 125. Additionally, insome embodiments each of metal layers 930, 990, and 950 may include abias tab 985 that is configured to be coupled with, and receive avoltage bias from, a DC power source. In some embodiments, the collectorlayers 915 or 916 may include or be coupled with a sub-collector layer,as described above with reference to collector layer 215. In someembodiments, the number of different elements may be different thandepicted in FIG. 9. For example, in embodiments the compound varactor900 may include more or fewer collector layers, base layers, or metallayers. Additionally, in some embodiments the relative sizes of elementsmay be different than depicted in FIG. 9. For example, in someembodiments the metal layer 925 may be the same length or width as baselayer 920.

As can be seen in FIG. 9, the metal layer 995, which may be similar toone or both of metal layers 225 or 245, may be an element of bothvaractors 910 and 906, and configured to allow an RF signal to propagatefrom varactor 910 to varactor 906, or vice versa. Similarly, metal layer990, which may be similar to one or both of metal layers 230 or 250, maybe an element of both varactors 910 and 906, and configured to allow anRF signal to propagate from varactor 910 to varactor 906, or vice versa.

Therefore, as shown in FIG. 9, an RF signal may enter compound varactor900 at metal layer 930, where it may propagate through varactor 905 tocollector layer 915. From collector layer 915, the RF signal maypropagate through varactor 910 to metal layer 990 to varactor 906. TheRF signal may then similarly propagate through compound varactor 901 tometal layer 950, where it may then exit system 900. It will beunderstood that this description of how an RF signal may propagatethrough compound varactor 900 is only intended as an example, and inother embodiments the RF signal may enter, exit, or propagate throughdifferent layers or in a different direction dependent on the specificconstruction of the compound varactor 900 or a circuit utilizingcompound varactor 900.

FIG. 10 depicts an alternative embodiment that includes a compoundvaractor 1000. Elements of compound varactor 1000 may be similar tocompound varactor 200 or compound varactor 900, and be numberedsimilarly.

Varactor 1005 may include collector layer 1015, base layer 1020, metallayer 1025, and metal layer 1030, which may be respectively similar tocollector layer 215, base layer 220, metal layer 225, and metal layer230. In embodiments, vias 1035, which may be similar to vias 235, mayelectrically connect one or more of base layer 1020, metal layer 1025,and metal layer 1030. Varactor 1010 may include collector layer 1015 andbase layer 1040, which may be respectively similar to collector layer215 and base layer 240. Varactor 1010 may further include metal layer1095 and metal layer 1090, which will be described in greater detailbelow. Vias (not labeled for the sake of clarity) may electricallyconnect base layer 1040, metal layer 1095, and metal layer 1090.

Varactor 1006 may include collector layer 1016 and base layer 1021,which may be respectively similar to collector layer 215 and base layer220. Varactor 1006 may further include metal layers 1095 and 1090, asdescribed in further detail below. Vias 1035 (not labeled for the sakeof clarity) may electrically connect one or more of base layer 1021,metal layer 1095, and metal layer 1090. Varactor 1011 may includecollector layer 1016 and base layer 1041, which may be respectivelysimilar to collector layer 215 and base layer 240. In embodiments,varactor 1011 may further include metal layer 1096 and metal layer 1091,as described in further detail below. In embodiments, vias 235 (notlabeled for the sake of clarity) may electrically connect one or more ofbase layer 1041, metal layer 1096, and metal layer 1091.

Varactor 1007 may include collector layer 1017 and base layer 1022,which may be respectively similar to collector layer 215 and base layer220. Varactor 1007 may further include metal layers 1096 and 1091. Vias1035 (not labeled for the sake of clarity) may electrically connect oneor more of base layer 1022, metal layer 1096, and metal layer 1091.Varactor 1012 may include collector layer 1017, base layer 1042, metallayer 1047, and metal layer 1052, which may be respectively similar tocollector layer 215, base layer 240, metal layer 245, and metal layer250. In embodiments, vias 1035 may electrically connect one or more ofbase layer 1042, metal layer 1047, and metal layer 1052.

In some embodiments the collector layers 1015, 1016, and 1016 may becoupled with a bias input 1080 that may be coupled with and configuredto receive a voltage bias from, for example, DC power source 125.Additionally, in some embodiments each of metal layers 1090 and 1091 mayinclude a bias tab 1085 that is configured to be coupled with, andreceive a voltage bias from, a DC power source. In some embodiments, thecollector layers 1015, 1016, and 1017 may include or be coupled with asub-collector layer, as described above with reference to collectorlayer 215. In some embodiments, the number of different elements may bedifferent than depicted in FIG. 10. For example, in embodiments thecompound varactor 1000 may include more or fewer collector layers, baselayers, or metal layers. Additionally, in some embodiments the relativesizes of elements may be different than depicted in FIG. 10. Forexample, in some embodiments the metal layer 1025 may be the same lengthor width as base layer 1020.

As can be seen in FIG. 10, the metal layer 1095, which may be similar toone or both of metal layers 225 or 245, may be an element of bothvaractors 1010 and 1006, and configured to allow an RF signal topropagate from varactor 1010 to varactor 1006, or vice versa. Similarly,metal layer 1090, which may be similar to one or both of metal layers230 or 250, may be an element of both varactors 1010 and 1006, andconfigured to allow an RF signal to propagate from varactor 1010 tovaractor 1006, or vice versa. Similarly, metal layer 1091, which may besimilar to metal layer 1090, may be an element of both varactors 1011and 1007, and configured to allow an RF signal to propagate fromvaractor 1011 to varactor 1007, or vice versa. Similarly, metal layer1091, which may be similar to one or both of metal layer 230 or 250, maybe an element of both varactors 1011 and 1007, and configured to allowan RF signal to propagate from varactor 1011 to varactor 1007, or viceversa.

Therefore, as shown in FIG. 10, an RF signal may enter the system 1000at metal layer 1030. The RF signal may propagate through varactor 1005to collector layer 1015, where it may then propagate back up throughvaractor 1010 to metal layer 1090. The RF signal may propagate throughmetal layer 1090 to varactor 1006 where it may propagate through thecompound varactor 1002 to metal layer 1091. From metal layer 1091, theRF signal may propagate to varactor 1007 to metal layer 1052 where itmay then exit the compound varactor 1000. It will be understood thatthis description of how an RF signal may propagate through compoundvaractor 1000 is only intended as an example, and in other embodimentsthe RF signal may enter, exit, or propagate through different layers orin a different direction dependent on the specific construction of thecompound varactor 1000 or a circuit utilizing compound varactor 1000.

Compound varactors 200, 201, 300, 501, 502, 503, 504, 601, 602, 603,800, 900, or 1000 may be incorporated into a variety of systems. A blockdiagram of an example system 1100 is illustrated in FIG. 11. Asillustrated, the system 1100 includes a power amplifier (PA) module1102, which may be a radio frequency (RF) PA module in some embodiments.The system 1100 may include a transceiver 1104 coupled with the PAmodule 1102 as illustrated. The PA module 1102 may include one or moreof compound varactors 200, 201, 300, 501, 502, 503, 504, 601, 602, 603,800, 900, or 1000. In various embodiments, the compound varactors 200,201, 300, 501, 502, 503, 504, 601, 602, 603, 800, 900, or 1000 mayadditionally/alternatively be included in the transceiver 1104 toprovide, e.g., up-converting, or in an antenna switch module (ASM) 1106to provide various switching functions.

The PA module 1102 may receive an RF input signal, RFin, from thetransceiver 1104. The PA module 1102 may amplify the RF input signal,RFin, to provide the RF output signal, RFout. The RF input signal, RFin,and the RF output signal, RFout, may both be part of a transmit chain,respectively noted by Tx-RFin and Tx-RFout in FIG. 11.

The amplified RF output signal, RFout, may be provided to the ASM 1106,which effectuates an over-the-air (OTA) transmission of the RF outputsignal, RFout, via an antenna structure 1108. The ASM 1106 may alsoreceive RF signals via the antenna structure 1108 and couple thereceived RF signals, Rx, to the transceiver 1104 along a receive chain.

In various embodiments, the antenna structure 1108 may include one ormore directional and/or omnidirectional antennas, including, e.g., adipole antenna, a monopole antenna, a patch antenna, a loop antenna, amicrostrip antenna or any other type of antenna suitable for OTAtransmission/reception of RF signals.

The system 1100 may be suitable for any one or more of terrestrial andsatellite communications, radar systems, and possibly in various 1100and medical applications. More specifically, in various embodiments, thesystem 1100 may be a selected one of a radar device, a satellitecommunication device, a mobile computing device (e.g., a phone, atablet, a laptop, etc.), a base station, a broadcast radio, or atelevision amplifier system.

Although the present disclosure has been described in terms of theabove-illustrated embodiments, it will be appreciated by those ofordinary skill in the art that a wide variety of alternate and/orequivalent implementations calculated to achieve the same purposes maybe substituted for the specific embodiments shown and described withoutdeparting from the scope of the present disclosure. Those with skill inthe art will readily appreciate that the teachings of the presentdisclosure may be implemented in a wide variety of embodiments. Thisdescription is intended to be regarded as illustrative instead ofrestrictive.

What is claimed is:
 1. A method comprising: depositing a collectorlayer; depositing a first base layer arranged in a first plurality ofparallel fingers directly onto the collector layer; and depositing asecond base layer arranged in a second plurality of parallel fingersthat are interleaved with the first plurality of parallel fingersdirectly onto the collector layer.
 2. The method of claim 1 wherein thecollector layer, the first base layer, and the second base layercomprise gallium arsenide, silicon, germanium, aluminum phosphide,aluminum arsenide, indium phosphide, gallium nitride, and combinationsthereof.
 3. The method of claim 1 wherein a first finger in the firstplurality of parallel fingers has a first width, and a second finger inthe second plurality of parallel fingers has a second width that is thesame as the first width.
 4. The method of claim 1 wherein a first fingerin the first plurality of parallel fingers has a first width, and asecond finger in the second plurality of parallel fingers has a secondwidth that is different than the second width.
 5. The method of claim 1wherein the first plurality of parallel fingers comprise a firstvaractor diode and the second plurality of parallel fingers comprise asecond varactor diode.
 6. The method of claim 1 further comprisingdepositing a sub-collector layer on a side of the collector layer thatis opposite the first base layer.
 7. The method of claim 1 furthercomprising depositing a sub-collector layer on a side of the collectorlayer that is opposite the second base layer.
 8. The method of claim 1further comprising depositing a first metal layer directly onto thefirst base layer on a side of the first base layer opposite thecollector layer.
 9. The method of claim 8 further comprising depositinga second metal layer on a side of the first metal layer opposite thefirst base layer.
 10. The method of claim 9 further comprising forming aplurality of vias at locations that electrically couple together thefirst metal layer, the second metal layer, and the first base layer. 11.The method of claim 9 wherein the first metal layer and the second metallayer comprise titanium, platinum, gold, zinc, nickel, beryllium, andcombinations thereof.
 12. A method comprising: fabricating a firstvaractor pair by: depositing a first collector layer; depositing a firstbase layer arranged in a first plurality of parallel fingers directlyonto the first collector layer; and depositing a second base layerarranged in a second plurality of parallel fingers that are interleavedwith the first plurality of parallel fingers directly onto the firstcollector layer; and fabricating a second varactor pair by: depositing asecond collector layer; depositing a third base layer arranged in athird plurality of parallel fingers directly onto the second collectorlayer; and depositing a fourth base layer arranged in a fourth pluralityof parallel fingers that are interleaved with the third plurality ofparallel fingers directly onto the second collector layer, wherein thefirst base layer is electrically coupled with the third base layer, andthe second base layer is electrically coupled with the fourth baselayer.
 13. The method of claim 12 wherein the first collector layer, thesecond collector layer, the first base layer, the second base layer, thethird base layer, and the fourth base layer comprise gallium arsenide,silicon, germanium, aluminum phosphide, aluminum arsenide, indiumphosphide, gallium nitride, and combinations thereof.
 14. The method ofclaim 12 further comprising depositing a first sub-collector layer on aside of the first collector layer that is opposite the first base layer.15. The method of claim 12 further comprising depositing a secondsub-collector layer on a side of the second collector layer that isopposite the third base layer.
 16. The method of claim 12 furthercomprising: depositing a first metal layer directly onto the first baselayer on a side of the first base layer opposite the first collectorlayer; depositing a second metal layer on a side of the first metallayer opposite the first base layer; and forming a plurality of vias atlocations that electrically couple together the first metal layer, thesecond metal layer, and the first base layer.
 17. The method of claim 16wherein the first metal layer and the second metal layer comprisetitanium, platinum, gold, zinc, nickel, beryllium, and combinationsthereof.
 18. The method of claim 12 further comprising: depositing athird metal layer directly onto the third base layer on a side of thethird base layer opposite the second collector layer; depositing afourth metal layer on a side of the fourth metal layer opposite thefourth base layer; and forming a plurality of vias at locations thatelectrically couple together the third metal layer, the fourth metallayer, and the third base layer.
 19. The method of claim 18 wherein thethird metal layer and the fourth metal layer comprise titanium,platinum, gold, zinc, nickel, beryllium, and combinations thereof. 20.The method of claim 12 wherein the first plurality of parallel fingersand the third plurality of parallel fingers have a first orientation,and the second plurality of parallel fingers and the fourth plurality ofparallel fingers have a second orientation that is opposite the firstorientation; and wherein the first plurality of parallel fingers and thefourth plurality of parallel fingers are opposite one another, and thesecond plurality of parallel fingers and the third plurality of parallelfingers are opposite one another.